Building a bike engined car has been a twinkle in my eye for a while now. I've done much lurking on this site following a couple different builds, notably Moti's work of art and Peter's MNR build. You could say mine started almost two years ago when I bought the donor ZX12. Last summer I bought a donor Miata and only this winter has design and construction started in earnest.

Why a kit? Honestly, I know the limits of my ability, and unlike others on this site, I am not a craftsman. I would like the project completed in a reasonable amount of time, and I want it to function properly and look good when it's done. I like MNR's frame - the round tubing allows them to have a stiff frame at a lower weight compared to others. I also liked the body style and other options like the lighting they offer.

Why a ZX12? I wanted something with a fair amount of power. It basically came down the the ZX12, ZX14 or obviously the Hayabusa. I looked around for a while looking for a good deal on a crashed bike and found one in a '12. Now that I'm as far into it as I am, and that I'm considering how to make more power (head work, bigger pistons, turbo etc) I would have probably been better off with a 'busa. But here I am.

The big news about my build is I'm mating the ZX12 engine to a Miata transmission. This may sound familiar as the folks at MotoIQ have been mating a 'busa to a miata trans for use in a miata, dubbed the Miatabusa. I realize the merits of this have been hotly debated all around the internet. Here's my take:

Why use a bike motor with a car trans? The bike motor is lighter than the car motor, it makes a sweeter sound, you get a clutch that's easy to slip and you don't have to worry about frying, you get reverse.

Why not? It's quite expensive, the car transmission weighs way more than the bike's, there are other ways to get reverse, you lose the sequential trans.

For me, the sound of a high revving bike engine evokes an emotion unlike any other. I just don't have the words to describe it. It just has to be a bike engine. Why a car trans then? If I was building a track car I might have thought twice about ditching the bike's trans. But this is going to be a street / track car. I plan on driving it as much as possible and therefore I want it to drive like a road going car. Lastly, since building from a kit is a little bit of a cop-out, I wanted to do something that actually required a lot of work, thinking, ingenuity and originality. Hence the car trans.

What's the status you ask? The kit is ordered. I called MNR last week and they said it should ship in a couple weeks. So I'm hoping to have it by the end of April. I have all the custom parts made and fitted to the engine. That required splitting the case to pull out the bike's transmission and patching up various things. Next step is to mate the transmission and get the engine running again to make sure the rebuild went OK.

Now for the engine! Last summer I removed it from the bike and got it started. The point was to twofold. One, I wanted to evaluate it's condition, and two, I wanted to make sure I had all the important bits off of the bike so I could part out the rest of it.

Really the first step in designing the adapter to the miata trans was to reverse engineer the clutch interface on the bike. For some reason, and all I can come up with is aesthetics, many sports bikes' clutch cover faces are at an odd angle to the shaft on which the clutch rides. This does not make it easy to reverse engineer.

To the machine shop!

Here the engine is on a CMM - a super accurate 3D measuring device. The shop is NRL, just over the bay bridge in Stevensville, MD. They did the work for free so long as I let the owner have a drive when it was done. What a deal!

I'm going to attempt to give y'all a run down of the design of the adapter. Honestly, it's kind of hard to explain. But I'll do my best. Although I'm doing things a little differently, the first post in the Miatabusa build actually does a pretty good job of explaining the basic idea.

What you're looking at here is the bike engine with the clutch cover off. The clutch assembly has been partially disassembled. What remains is the inner hub and the ring gear. That ring gear is really the key. It's driven directly by the crank. The whole clutch assembly rides on a shaft which is supported by bearings in the crankcase. This shaft runs inside the motor and is the input shaft of the bike's transmission. It has half of the gears on it.

The idea is to somehow hijack the ring gear and attach it to a shaft that, on one end, looks like the miata's crankshaft so a miata flywheel can bolt up. We have to have some means of supporting our new shaft. We also need to bolt up the miata transmission so we need an adapter plate. The solution is to make a new clutch cover. The cover will do a number of things 1) it will seal the crankcase as the original cover did, 2) it will not only support our new shaft, it will make sure it is aligned properly, 3) it will support the adapter plate so the transmission can bolt up, and in the correct place. We also need a shaft. The ring gear will mount to the shaft so we can get power from the crankshaft. One end will pass through the clutch cover and it will have the bolt pattern for a miata flywheel. The other end will pilot into, and be supported by, a bearing in the crankcase.

Here's a picture of the clutch cover. It's heading into the oven to get a little bigger so the bearings will drop in. You can see how the main bore is at an angle to the sealing face.

The bearings slipped right in. There's a snap ring to hold them in place. The bearings were actually quite an adventure to size properly. The thing is, when you disengage the clutch they see an enormous axial load. They also spin relatively fast and need to take a decent radial load as well. That assembly can get pretty hot too, so lubrication is a challenge. I ended up with a rather large pair of angular contact bearings. They are a matched pair that, when clamped together, a preload is generated that keeps them stiff and within their operating limits as the loads change. The large round raised lip is to locate the transmission adapter plate. Those six holes are to bolt the plate on.

This is the best shot I have of the shaft. See how one end is flanged with a bolt pattern? The miata flywheel will bolt of there. At the far other end, the end that's up in the air, that pilots into a bearing in the engine's crankcase. The large plain surface just below that is where the ring gear mounts. The threads below that are used with a nut to clamp the bearings. And the bearings ride on the shorter plain surface just below the threads. On the housing, you can see there is an oil seal as well.

Here's a look inside the ZX12s crankcase. The engine is upside-down. I've pulled off the oil pan as well as the bottom half of the crankcase. The shaft nearest the camera, the one with the counterweights is the crankshaft. If you look carefully you can actually see the bottoms of the pistons. The other two shafts are the transmissions shafts. On the far one, on the left, you can see the sprocket that would drive the bike's chain. The other trans shaft, the one with the smaller gears, is normally driven by the bike's clutch. Look at the right end of that shaft, there is a small spline there. That's about where the ring gear sits. You can see the gear on the crank that drives the ring gear.

You can see the bottom half of the crankcase as well as the two transmission shafts removed.

Here is the crankcase with the transmission removed.

This one is fairly illustrative. You can see the gear on the crank as well as the ring gear on my shaft. You also start to see how the adapter plate will fit and the flywheel will bolt up.

Here is the engine back together and the adapter mounted. I had another machine shop do this work: Complex Manufacturing in Union Bridge, MD. They did an amazing job on all the work: the fits between the bearings and the housing, the bearings and the shaft, and the housing on the crankcase. Everything fit perfectly. I am able to blind-mate my assembly on to the engine with absolutely zero binding, nothing binds as I tighten the cover down, and I have the tiniest amount of backlash between the ring gear and the crank - perfect!

I forgot to mention one of the cornerstone's of the whole thing. If you've been paying attention you might be wondering how I mounted the ring gear to my shaft. The answer: With this beauty:

This is a "clamping set". Basically an inner and outer hub that screw together. They are both slotted and their interface to each other is tapered. So as you screw them together the taper forces the outer hub to get bigger and the inner hub to get smaller. The inner hub slides over my shaft and the outer hub slides into the ring gear. It just so happened that the ring gear's ID was machined to a stock size for one of these clamping sets. Will it work? I think (hope) so. Firstly, the set is rated for 570 ft-lbs of torque. The engine makes about 91 ft-lbs and there is that primary reduction of 1.6. So the set will see 146 ft-lbs. That gives me a lot of wiggle room - but there can be fairly high shock loads which are hard to estimate. If I decide to rev up the motor and dump the clutch, for instance. So we'll see. Secondly, the manufacturer supplies clamping pressure data and the resulting connection is extremely strong - as strong as a very tight interference fit - the kind you need temperature differentials and a press to get together. Any stronger and you start to run into problems with the pressure yielding the shaft or ring gear. So we'll see. If this doesn't work I may be forced to convert things over to a splined connection.

So what's next? As I alluded to earlier, lubrication for my bearings was going to be a challenge. For simplicity's sake I wanted to use sealed bearings. But, for the load, temperature and speeds they were going to see, I just couldn't make that work. So I ended up with an open set of bearings that requires external lubrication. Luckily, the bike engine has a lubrication system. Even luckier, I removed the transmission shafts which required lubrication. My plan is to hijack one of the oil "lines" that went to the bike's transmission shafts and direct it to my bearings. This will require some AN3 hard brake line and various fittings. I bought some of it already and tried something. Turns out I need a couple more fittings I didn't order the first time. They should be here tuesday maybe. I'm on vacation this weekend, and for 10 days later in the month. So work may be slow for a little while. If I'm really hardcore I may be able to get the oiling set up and the engine running again before I have to leave.

Yeah the machine shop did a great job. There is a fair amount of NRE (non-recurring engineering) that goes into programming a CNC machine for parts like these - which I've already paid for. So for another copy? $4,000? I'll have to ask the machine shop.

I was able to install the flywheel, clutch and pressure plate on my housing today.

And I fit it up to the miata trans. It fits! I went through and measured the depths of the pilot bearing and clutch disk splines. And although you can't see it when bolted up to the transmission, everything should be at the proper depth.

I did manage to mess something up though. In the adapter plate I put tapped holes that go into pin holes on the transmission. And I neglected to put tapped holes where two belong. AND I used the wrong thread pitch so I can't reuse the standard bolts. Turns out the miata uses a fine thread and I called out the standard size which is one step more coarse. Not that big of a deal. I can just order new bolts and drill a couple through holes. I'm just thankful nothing more important is out of whack.

Eggsellent write up (it's almost Easter you know). Even I can follow it.

One of the issues of adapting a bike engine to a car tranny is the extra power it takes to turn that (comparatively) ginormous flywheel and tranny so couldn't you use a smaller flywheel and clutch assembly? Get something half way between a bike size (which won't work- long) and the overweight car standard assembly.

One of the issues of adapting a bike engine to a car tranny is the extra power it takes to turn that (comparatively) ginormous flywheel and tranny so couldn't you use a smaller flywheel and clutch assembly? Get something half way between a bike size (which won't work- long) and the overweight car standard assembly.

Very true. The additional inertia in the flywheel is regrettable. By car standards the miata's assembly is already pretty small and I focused on the miata options because that's the transmission I was using. But I could have expanded my options to anything that had the proper spline really. I bought setup for a 1.8l miata. The 1.6l stuff is slightly smaller so that is an option that I didn't go with for a reason I can't remember. There is a new low MOI twin disk setup being offered by 949racing. A single disk that holds 250ftlbs weighs 11.7 pounds and the twin will hold 400 ft lbs and weighs 14 pounds on a 7.25" diameter. By comparison, aluminum flywheel with ACT clutch and pressure plate weighs in at 21.6 lbs on a 8.46" diameter! Not low cost, but with the weight and inertia savings, probably wide grin

I wanted to share part of the analysis of miata trans vs. bike trans. I think the best way to compare the two options from a performance standpoint is to look at how much torque each puts to the ground. This is pretty easy to calculate:Flywheel Torque X Transmission Ratio X Differential Ratio = Torque at wheel.Do this for each gear in the transmission and plot it vs. road speed. Why road speed? It's a good common basis on which to compare the options. You can't just use engine speed because it's the transmissions we are trying to compare. And using torque after the transmission isn't the whole picture because the diff is in there. Also, it doesn't mean a whole lot so it's hard to think about. I calculate road speed using a typical 205/50 R15 which is pretty standard for a car like this.

So here we have each gear of the bike trans vs. the miata trans using a standard miata 4.1 diff.The bike trans is in indigoish and the miata's in brownish. Comparing the bike to the miata, we see the bike's gears are much closer together. This keeps the engine in the powerband, which is why we see the miata's curves dip below the bike's at the beginning of each shift. But, since they are so close together, the bike's gears top out at 114 MPH. And 1st gear is a much taller ratio so it will be harder to launch. On the other hand, through the meat of the speed range, with the bike gears you get slightly more torque at the expense of having to shift more often.

That top speed is a real bummer though. Luckily, there are numerous ratios for the miata diff housing. The tallest seems to be 3.636 available in '79-'82 Mazda 626. This is a fair bit taller for the bike's transmission which has the effect of pushing the curves down and to the right. This time, each shift of the miata transmission gives up a little more torque to the bike and the bike is able to achieve a higher top speed than it was before, about 128 mph. Although 1st gear is compromised more, for such a lightweight car it's probably not a big deal. Am I planning on spending a lot of time above 128mph? Not really. I am planning on spending some time up there though. And for a road-going car I don't want to have to be shifting constantly. So, it's a miata 5speed with a standard 4.1 diff. I give up little pockets of torque, but it's really not that much, and I have a decent launch gear and a power-limited top speed.

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